There are occasions when it is necessary to remove a length of tubing which has been fixed in place in a borehole. This tubing may be borehole casing which is surrounded by cement. Sometimes such removal of a length of tubing is done in preparation for setting a cement plug when a well is being abandoned. Removing a length of tubing which has been fixed within a borehole is customarily done with a rotary milling tool, customarily referred to as a section mill or casing mill, which comminutes the tubing to swarf.
Rotary milling tools frequently have a tool body and a plurality of cutting assemblies projecting from or extensible from the tool body and distributed azimuthally around a longitudinal axis of the tool body, wherein each cutting assembly comprises a steel supporting structure and a plurality of cutters with cutting surfaces made of a harder material, which may be sintered tungsten carbide.
It is normal that the rotation of the tool is started with little or no weight on the tool and then weight is applied, pushing the tool axially downwards into contact with the tubing and thereby starting the milling operation in which the tool cuts while driven in rotation and urged axially forward by the weight on the tool.